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Magnatone Custom M10

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  • #91
    thank you, mister

    SPKR 8" OXFORD 80L5N-1 16 Ohm 14oz AlNiCo// 465-435 [WEEK 35 1964] // 25-1016

    however itís missing the 3" McGREGOR Tweeter 25 1015 // 789434 9 [WEEK 34 1964] as it was blown

    Comment


    • #92
      Originally posted by TelRay View Post
      SPKR 8" OXFORD 80L5N-1 16 Ohm 14oz AlNiCo// 465-435 [WEEK 35 1964] // 25-1016

      however it’s missing the 3" McGREGOR Tweeter 25 1015 // 789434 9 [WEEK 34 1964] as it was blown
      Ah, an oldie-but-goodie. For some reason I thought you might have had a current production speaker there, something I could find "on the rack". No matter, and don't sweat the missing tweeter. It would just distract you with tinny racket from an overdriven amp. The Oxford delivers a gutsy sound, plenty impressive. With you playing thru it, of course!

      I have an M13 in the shop awaiting my attention. Way too many knobs!
      Enjoy. Every. Sandwich.

      Comment


      • #93
        Originally posted by Leo_Gnardo View Post
        Ah, an oldie-but-goodie. For some reason I thought you might have had a current production speaker there, something I could find "on the rack". No matter, and don't sweat the missing tweeter. It would just distract you with tinny racket from an overdriven amp. The Oxford delivers a gutsy sound, plenty impressive. With you playing thru it, of course!

        I have an M13 in the shop awaiting my attention. Way too many knobs!
        On the M13, don’t be surprised if the schematic doesn’t match the amp exactly. On my M13A, I found significant changes, even though the amp appeared to be original from the factory. It had the schematic stapled inside the cabinet too.

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        • #94
          Hi y'all,
          dropping a quick note to know (and wish) everyone is OK in these crazy times
          be safe!

          Comment


          • #95
            Hi all,

            I have recently been watching some videos from Uncle Doug on youtube... and I like this guy, very instructive!

            I was specially interested in the detailed method description to calculate power dissipation on each output tube (measure resistance through the output transformer to the output tubes plate, measure voltage drop between the same points and plate voltage to ground to calculate the power dissipation)
            so I decided to try that on my amp.

            With the amp turned off and disconnected from the mains I measured the RESISTANCE from point “A” to each PIN 7 on the POWER TUBES (marked in RED in the schematic below)

            R1: 154.7 Ohm
            R2: 243.5 Ohm

            The issues appeared when I turned ON the amp and measured VOTAGE DROP across the same points, I could simply not achieve a STABLE VOLTAGE READING. Using the meter’s AVG MAX and MIN functions I was able to measure:

            Vdrop1: (1.9 +/- 1.0) V
            Vdrop2: (3.8 +/- 1.2) V

            The PLATE VOLTAGE (PIN 7 to GROUND is also changing by a range of 2 V)

            Vplate1: (384 +/- 2) V
            Vplate2: (382 +/- 2) V

            Trying to understand where the fluctuation was coming from I checked (measured 47 and 51 nF) and replaced by new ORANGE DROPS, the 2x 47nF Capacitors (marked in BLUE in the schematic below). NO CHANGE

            I discovered that the rate at which the VOLTAGE DROPS are fluctuating is related to the SPEED of the VIBRATO FUNCTION. However the variation exists with the EFFECT turned ON and OFF (I had to add an external FOOTSWITCH to turn it OFF).



            LINK to full SCHEMATIC: https://www.magnatoneamps.com/schema...natone_M10.pdf

            REMINDER: all electrolytic caps have been replaced with new ones

            Any ideas (sure you have some)?
            Last edited by TelRay; 04-17-2020, 01:37 AM.

            Comment


            • #96
              I'd think the trem is not turning fully off. Try removing that last 12DW7 tube (phase inverter). See if you get a stable reading.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

              Comment


              • #97
                good call, the readings are indeed stable with the PI 12DW7 pulled out of the circuit AND the tremolo OFF (unstable if it's turned ON)

                I get now:

                Vdrop1: 2.4 V
                Vdrop2: 2.9 V

                Vplate1: 378.5 V
                Vplate2: 377.2 V

                POWER 1 = 2.4 V / 154.7 Ohm * 378.5 V = 5.9 W
                POWER 2 = 2.9 v / 243.5 Ohm * 377.2 V = 4.5 W

                QUESTIONS:
                1) what do you think about the tubes being too cold for a maximum power dissipation of 13.2 W for these 7189As?
                2) what about the difference of 30% between the two tubes?
                3) is the "trem not turning fully off" a normal thing given the design of this circuit or something that needs to be fixed?

                thx!

                Comment


                • #98
                  I'm not that familiar with the Magnatone's so not sure about the tremolo thing. Can you still hear it when it's turned off via footswitch while you are playing? If not, I don't think it's a problem.
                  The difference between power tubes could create hum, if no hum issues then they should be fine. Same with the cold bias, no issue if it sounds good.
                  If you wanted to tweak the idle current, you don't have adjustable bias so you would have to alter the bias circuit or decrease the cathode resistor.
                  "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                  Comment


                  • #99
                    Originally posted by g1 View Post
                    I'm not that familiar with the Magnatone's so not sure about the tremolo thing. Can you still hear it when it's turned off via footswitch while you are playing? If not, I don't think it's a problem.
                    no sir, I cannot hear it at all. so I guess I'l leave it like that unless someone else jumps in


                    Originally posted by g1 View Post
                    The difference between power tubes could create hum, if no hum issues then they should be fine.
                    yepp, that's exactly why I started looking into this matter, there is some 60 Hz humming with nothing connected and not affected by volume or tone controls

                    Originally posted by g1 View Post
                    Same with the cold bias, no issue if it sounds good.
                    If you wanted to tweak the idle current, you don't have adjustable bias so you would have to alter the bias circuit or decrease the cathode resistor.
                    in that case what makes sense to do first? I assume changing the cathode resistor value to increase POWER and then try balancing the TUBES
                    Guidance on how to do both will be appreciated

                    For what I've seen the 7189A are not widely available so I might not be really super interested in pushing them much. The question is that maybe running them at less than 50% is too low (unless the Magnatone guys had the power section over engineered on purpose)


                    thx!
                    Last edited by TelRay; 04-17-2020, 03:10 AM.

                    Comment


                    • IIRC, the amp has both fixed and cathode bias. To increase idle current lower the absolute value of grid bias by increasing the 22k resistor that connects to the bias filter cap.

                      there is some 60 Hz humming with nothing connected and not affected by volume or tone controls
                      Unbalanced output tubes will introduce 120Hz ripple (not 60Hz) hum.

                      To allow for balancing you may use 2 separate cathode resistors of around 200R, but one lower than the other as required.
                      Or you may use a 50R resistor to ground + a 200R/2W balancing pot on top, wiper connected to 50R, outer pot lugs to the cathodes.
                      - Own Opinions Only -

                      Comment


                      • thank you very much!

                        Originally posted by Helmholtz View Post
                        IIRC, the amp has both fixed and cathode bias. To increase idle current lower the absolute value of grid bias by increasing the 22k resistor that connects to the bias filter cap.
                        to re-cap, it's been written that the amp has a "partial cathode bias" (post#51) or "hybrid bias" (post #69), I understand that "both fixed and cathode bias"... they all mean the same, right?
                        the bias circuit is currently as the original schematic

                        as a reminder, the POWER TRANSFORMER was supplying a -67 V bias voltage that we had to bring down to the nominal -17V by adjusting the 12K R and to -32 V by tweaking the 120 K R.



                        now we are going to modify the 22K resistor to increase the BIAS idle current (this one inside the red rectangle)



                        (pardon the maybe basic question) is this change (22K R) going to only modify the CURRENT (mA) or also change the VOLTAGE at point "F". If so, should then I play again with the value of the (old value) 12K R to maintain -17V at point "F"?


                        Originally posted by Helmholtz View Post
                        Unbalanced output tubes will introduce 120Hz ripple (not 60Hz) hum.
                        To allow for balancing you may use 2 separate cathode resistors of around 200R, but one lower than the other as required.
                        Or you may use a 50R resistor to ground + a 200R/2W balancing pot on top, wiper connected to 50R, outer pot lugs to the cathodes.
                        like this?




                        thx!!!

                        Comment


                        • it's been written that the amp has a "partial cathode bias" (post#51) or "hybrid bias" (post #69), I understand that "both fixed and cathode bias"... they all mean the same, right?
                          Yes, it's a combination of fixed and cathode bias.

                          is this change (22K R) going to only modify the CURRENT (mA) or also change the VOLTAGE at point "F". If so, should then I play again with the value of the (old value) 12K R to maintain -17V at point "F"?
                          To increase idle current (and thus plate dissipation), you need a smaller negative voltage (e.g. -15V instead -17V) at point "F". (Actually -15V would be a higher = more positive voltage than -17V, that's why I wrote "lower absolute value of grid bias" above). This can be achieved by increasing the value of the 22k resistor.

                          The picture with the 2 separate cathode resistors shows what I meant. The tube with the lower current will require a lower cathode resistor for balance. This can be achieved by paralleling an additional resistor. But the balancing pot would be a more elegant and versatile solution.

                          Do you have 60Hz or 120Hz hum?
                          - Own Opinions Only -

                          Comment


                          • Originally posted by Helmholtz View Post
                            Yes, it's a combination of fixed and cathode bias.
                            verstandeden!

                            Originally posted by Helmholtz View Post
                            To increase idle current (and thus plate dissipation), you need a smaller negative voltage (e.g. -15V instead -17V) at point "F". (Actually -15V would be a higher = more positive voltage than -17V, that's why I wrote "lower absolute value of grid bias" above). This can be achieved by increasing the value of the 22k resistor.
                            very clear

                            Originally posted by Helmholtz View Post
                            The picture with the 2 separate cathode resistors shows what I meant. The tube with the lower current will require a lower cathode resistor for balance. This can be achieved by paralleling an additional resistor. But the balancing pot would be a more elegant and versatile solution.
                            I like elegant

                            Originally posted by Helmholtz View Post
                            you have 60Hz or 120Hz hum?
                            good question... I think I have both



                            my reasoning is that it is 60 Hz, yes. Because I see 60, 120, 180, 240, etc
                            But... if it was purely 60 Hz then the 120 should be lower in intensity, right? additionally the harmonics from the 120 Hz Hum re inforce the ones of the 60 Hz and I see re-inforced peaks at 240 Hz, etc
                            so i believe there's also some 120 Hz which should be coming from the unbalanced tubes.
                            do you agree?

                            Comment


                            • hi all, hope you guys are still healthy and safe

                              i've been investigating the issue a bit more and found out through a suggestion from a friend that i have not checked the 270 K resistors. oh, surprise, one of them was not great at 347 K (+29%) and the other was marginally OK at 297 K (+10%). So, both replaced by nice 1% tolerance resistors.

                              Also double checked the 1 K and 1 Ohm resistors, all OK

                              Re-checked POWER DISSIPATION and got:

                              POWER 1 = 6.4 W
                              POWER 2 = 4.9 W


                              still too cold and 30% unbalanced

                              changed the 22 K to a 34 K R following previous suggestions and got a bias of -13.0 V

                              POWER 1 = 9.4 W
                              POWER 2 = 7.7 W


                              I want to be a bit more conservative than the 9.4 W with this tubes as I've seen they are not easy (or cheap) to find and. the unbalance was better at 22%

                              at that point I decided to swap the tubes (7189A tube 1 went to socket #2 and viceversa)

                              POWER 1 = 7.5 W
                              POWER 2 = 8.5 W


                              still marginally hot (for my conservative approach) but the balance was at 13%

                              changed to a 30.7 K resistor and -13.8 bias

                              POWER 1 = 6.9 W
                              POWER 2 = 7.7 W


                              Now the difference is at around 12% and I think running the tubes at about 60% of max power will give them many years of happiness

                              After that I connected a decal box and tried increasing resistance and power to hear how much "punch" i was missing because of this conservative approach and have not really noticed a difference.

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